1. Field of the Invention
The present invention relates to a ceramic circuit board for mounting electric parts with a high density, more particularly to a ceramic circuit board which is free from short-circuit due to solder-flow or bonding defects thereby to have an excellent operating reliability, and is capable of being easily mass-produced with a high production yield.
2. Description of the Related Art
Conventionally, there has been widely used a ceramic circuit board which is manufactured by integrally bonding a metal circuit plate having an electrical conductivity onto a surface of a ceramic substrate such as alumina (Al.sub.2 O.sub.3) sintered body having an excellent insulating property through a bonding layer such as a brazing material, an adhesive agent, or a metallized layer. Such ceramic circuit board has been widely used as a substrate for a power-transistor module and a switching power source module.
However, in case of such ceramic circuit board, since inclusions (bonding layers) such as brazing material, adhesive agent, or the metallized layer exist between the metal circuit plate and the ceramic substrate, a heat resistance between the metal circuit plate and the ceramic substrate would become large. As a result, there has been posed a problem such that it is difficult to smoothly release and radiate a heat generated at semiconductor element provided onto the metal circuit plate so as to remove the heat outside from a circuit system.
In order to eliminate the afore-mentioned problem, in recent years, there has been reviewed and tried a method (DBC method) comprising the steps of: blanking a plate member to form a metal circuit plate having a predetermined circuit pattern; contacting and disposing the metal circuit plate onto the ceramic substrate to form a laminated body; and only heating the laminated body thereby to directly bond the metal circuit plate onto the ceramic substrate without using the bonding layer such as the brazing material layer, adhesive agent layer or metallized layer. That is, the direct bonding method is a method of directly bonding a metal to a ceramic without intervening any bonding layer such as brazing material layer, adhesive layer or metallized layer between the two members.
In this direct bonding method, an eutectic liquid phase consisting of a bonding agent and a metal is generated. The bonding agent (e.g. oxygen in a case where the metal is copper) is contained in the metal or exists on a surface of the metal. This eutectic liquid phase increases wettability of the ceramic substrate whereby both members of the ceramic substrate and the metal are directly bonded to each other.
On the other hand, in order to cope with high-integration and high-power operation of a semiconductor element to be mounted on the circuit board, there has been widely used a ceramic circuit board using an aluminum nitride (AlN) substrate which is excellent in heat radiating property, because AlN substrate has a higher thermal conductivity in comparison with other ceramic substrates such as a conventional alumina (Al.sub.2 O.sub.3) substrate.
That is, an aluminum nitride (AlN) circuit board is manufactured by a method comprising the steps of: preparing an aluminum nitride (AlN) substrate having a higher thermal conductivity than the conventional ceramic substrates such as alumina (Al.sub.2 O.sub.3) substrate or the like; bonding a metal circuit plate, for example a copper circuit plate on a surface of the aluminum nitride substrate by direct bonding method; thereafter, solder-bonding parts such as semiconductor element, resistor, capacitor or the like on a surface of the metal circuit plate. Thus manufactured aluminum nitride circuit board has also come into wide use recently as a semiconductor element mounting board.
However, in recent years, technical demands for realizing miniaturization of electrical devices using the semiconductor mounting board has been further increased, so that there has been a great demand for a high-densely mounted substrate on which more number of parts such as semiconductor element or the like are mounted. In this regard, in the high-denselyr mounted substrate, since the parts such as semiconductor element, resistor, capacitor or the like are required to be mounted on the same common substrate with a high density, it becomes an essential requirement to downsize and refine the the circuit layer for electrically connecting the parts to each other.
As a method of forming the downsized and refined circuit layer, there has been conventionally used a method of forming the circuit layer in which metal circuit plates such as copper plate or the like formed by press-working or etching treatment so as to have a predetermined shape are used. Further, there has been also widely used a circuit board which is manufactured by a method comprising the steps of: forming a fine circuit layer on a surface of the ceramic substrate by a metallizing method; and solder-bonding the parts such as semiconductor element or the like on a surface of the circuit layer thereby to integrally mounting the parts on the substrate.
However, in the conventional circuit board constituted as above, a peripheral portion of the metal circuit plate composed of copper or the like was formed to be plane or flat with no projection. Therefore, when the parts were solder-bonded to the surface of the circuit plate, the solder was liable to flow out toward a region other than a bonding portion, so that bonding defects of parts such as semiconductor element or the like were liable to occur, whereby there was posed a problem such that the fine circuit layer was short-circuited by the flowed-out solder. As a result, there has been also posed a problem such that a malfunction of the circuit board is liable to occur thereby to greatly lower the production yield of the circuit board.
In particular, in case of a circuit pattern where the parts such as semiconductor element or the like are obliged to be arranged and positioned to a portion close to a peripheral position of the metal circuit plate, the molten solder is liable to flow out to a portion between the adjacent circuit patterns thereby to cause solder-voids and many soldering-defects, so that there has been also posed a problem such that the production yield of the circuit board is greatly lowered.